•gst, 

University  of  California  •  Berkeley 


SEXTANT  WITH  ATTACHED  SPIRIT-LEVEL  HORIZON, 


'i;:        •'• 


SEXTANT  WITH  ATTACHED  SPIRIT-LEVEL  HORIZON. 

By  GEORGE  DAVIDSON,  Assistant  U.  S.  Coast  Survey. 


From  the  Journal  of  flic  Franklin  Institute. 

It  very  frequently  happens  that  the  traveler  and  the  navigator  are 
placed  in  situations  where  it  is  in  the  highest  degree  desirable  to  de- 
termine their  geographical  positions,  or  to  measure  the  elevation  of 
some  mountain  by  an  altitude  or  depression,  and  yet  the  means  at  their 
command  may  be  inefficient.  At  sea,  the  sextant  is  the  best  available 
instrument  for  measuring  angles  of  elevation  or  azimuth,  but  for  the 
former  purpose  it  can  be  used  only  when  the  object  and  sea  horizon 
are  both  visible.  Sometimes,  however,  when  it  is  very  desirable  to 
measure  an  altitude,  the  sun  is  so  low  that  the  bright  and  dazzling  re- 
flection from  the  surface  of  the  water  obscures  the  horizon ;  or  the 
horizon  is  hidden  by  a  low  fog  whilst  the  sun  is  visible  through  it. 

On  shore,  the  traveler,  whose  outfit  of  astronomical  instruments  is 
limited  to  a  pocket  chronometer,  sextant,  and  artificial  horizon,  finds 
constantly  annoying  occasions  when  his  means  fail  him  in  important 
determinations.  The  sun  may  be  too  high  for  observing  double  re- 
flections with  the  ordinary  sextant ;  the  sun  or  a  mountain  may  be  too 
low  to  admit  of  available  reflection  in  the  artificial  horizon ;  and  in 
particular  these  means  positively  fail  him  when,  from  any  elevated 
point,  he  wishes  to  measure  the  depression  of  some  object,  as  of  the 
sea  horizon,  by  which  to  determine  his  elevation  or  distance,  knowing 
one  of  them,  or,  knowing  the  distance  of  another  and  lower  mountain, 
to  determine  the  difference  of  elevation. 

We  have  encountered  all  these  difficulties,  and  also  the  less  frequent 
one  of  falling  in  with  a  reef  at  night  with  the  stars  visible  but  the  sea 
horizon  totally  obscured  in  darkness.  This  occurrence  first  directed 
our  attention  to  the  subject  of  adjusting  an  artificial  horizon  to  the 
sextant  about  twelve  years  since,  but  we  failed  to  solve  the  problem. 
Three  years  since,  when  daily  using  the  hand-level  generally  known 
to  the  trade  as  Locke's  level,  we  saw  the  means  at  our  hand  ready  for 
application,  and  fitted  it  to  a  Gambey  sextant. 

Plate  IV.  shows  this  level  in  detail.  Fig.  1  gives  the  general 
appearance  of  the  sextant  and  the  attached  level ;  Fig.  2  is  a  trans- 
verse section  through  the  bubble,  cross  wire,  and  reflector ;  Fig.  3  is 
a  longitudinal  section  through  the  bubble,  reflector,  and  double  convex 
lens.  The  tube  B  carrying  the  spirit-level  is  closed  at  each  end  with 


plane  glasses  GG;  the  half  of  the  tube  lengthwise  and  on  the  side 
nearer  the  face  of  the  sextant  is  unoccupied  from  end  to  end  to  per- 
mit vision  directly  through  the  plane  glasses  to  any  object  or  reflected 
object,  whilst  the  other  half  carries  the  reflector  R,  having  its  reflecting 
surface  towards  the  observer  and  placed  at  an  angle  of  45°  with  the 
axis  of  the  tube,  which  also  carries  the  half  or  segment  of  a  double 
convex  lens  L  secured  in  the  inner  tube  T,  which  is  capable  of  adjust- 
ment for  difference  of  vision  in  different  observers.  Towards  the  ob- 
ject end  of  the  tube  B,  over  an  opening  above  the  reflector,  is  secured 
a  small  spirit-level  8,  with  its  axis  parallel  to  the  tube,  and  having,  im- 
mediately beneath  the  tube,  an  adjustable  frame  F  carrying  a  fine  wire 
w  at  right  angles  to  the  axis  of  the  tube  and  parallel  to  the  horizontal 
plane.  The  images  of  the  bubble  of  the  spirit-level  and  of  the  cross 
wire  are  reflected  through  the  lens  to  the  eye  of  the  observer.  Other 
equivalent  means  may  be  employed  for  the  above  purposes,  such  as  a 
prism  for  the  reflector,  &c. 

For  observations  on  land,  where  the  instrument  may  be  held  with 
more  steadiness  than  at  sea,  or  even  secured  to  a  fixed  object,  a  small 
telescope,  such  as  the  weakest  power  usually  furnished  with  the  sextant, 
may  be  fitted  to  the  eye  end  of  the  tube,  and  thereby  afford  better  re- 
sults. 

We  have  affixed  the  level  to  the  sextant  by  having  a  tube  C,  about 
one  inch  in  length,  with  a  milled  head  M  and  exterior  screw  collar  to 
screw  into  the  telescope  holder  H  of  the  instrument,  and  then  sliding 
the  tube  B  into  it.  This  permits  the  use  of  the  usual  telescopes  by  re- 
moving the  tube  C. 

The  adjustment  of  this  level  is  made  either  by  changing  the  angle 
of  the  reflector,  or  moving  the  frame  carrying  the  cross  wire,  or  by 
elevating  one  end  of  the  level  itself;  and  consists  in  making  the  image 
of  the  cross-wire  bisect  the  image  of  the  bubble  when  a  distant  object 
in  the  same  horizontal  plane  and  seen  through  the  unoccupied  half  of 
the  tube  appears  on  a  level  with  the  image  of  the  cross-wire.  This 
adjustment  is  readily  effected  on  land,  may  be  made  by  means  of  the 
level  itself,  and  is  not  easily  deranged.  Should  it,  however,  become 
necessary  to  adjust  the  level  at  sea,  the  image  of  the  cross-wire,  when 
it  bisects  the  image  of  the  bubble,  is  made  to  appear  on  the  same  line 
with  the  visible  horizon  ;  and  the  correction  for  the  dip  of  the  horizon 
at  the  given  height  of  the  observer's  eye  is  applied  to  all  observed  alti- 
tudes and  depressions. 

'  The  index  error  may  be  determined  when  the  level  is  adjusted  on 
land  and  used  as  a  constant  quantity  for  a  not  very  extended  series  of 
observations,  or  it  may  be  determined  at  sea  whenever  the  horizon  is 
visible,  by  observing  the  depression  of  the  horizon  and  taking  the  dif- 
ference or  sum  of  the  observed  result  and  the  computed  dip  for  the 
index  error. 

The  operation  of  making  an  observation  for  the  altitude  or  depres- 
sion of  any  object  at  sea  or  on  land  is  as  follows :  Secure  the  level  in 
its  proper  place  on  the  instrument  and  hold  the  sextant  in  the  usual 
manner,  with  the  plane  of  its  face  in  the  vertical  plane  passing  through 


the  object  and  the  observer.  Looking  through  the  tube,  move  the  ver- 
nier arm  until  the  image  of  the  object  is  seen  through  the  unoccupied 
half  of  the  tube,  and  bring  that  image  into  the  same  horizontal  line 
with  the  image  of  the  cross-wire  at  the  time  it  bisects  the  image  of  the 
bubble,  and  if  necessary  note  the  time  by  chronometer.  If  the  sextant 
and  the  level  are  in  adjustment,  the  reading  on  the  arc  indicated  by  the 
vernier  is  the  observed  altitude  or  depression  of  the  object. 

At  night,  observations  may  be  made  by  illuminating  with  a  lamp  or 
by  chemical  means.  We  have  observed  the  altitude  of  the  moon  with- 
out any  artificial  light.  Prof.  Frazer  suggests,  as  a  chemical  means 
of  illumination,  a  small  tube,  containing  phosphorus  in  oil,  capable  of 
being  placed  over  the  level  at  pleasure,  and  a  small  quantity  of  air 
admitted  when  the  instrument  is  required  for  night  observations.  The 
length  of  the  bubble  of  the  unground  level  we  experimented  with  was 
two-tenths  of  an  inch,  but  should  be  shorter.  The  level  had  a  radius 
of  curvature  of  fifty-six  inches,  but  for  sea  observations  it  may  be 
smaller  for  bad  weather  and  until  practice  be  acquired. 

The  following  results  from  observations  for  latitude  are  given  as 
exhibiting  what  was  done  at  the  second  and  third  trials  with  the  in- 
strument. It  is  believed  that,  with  practice  and  a  level  adapted  for 
the  duty,  much  better  results  can  be  obtained,  especially  in  the  hands 
of  a  more  experienced  observer  with  the  sextant. 

Latitude  from  circummeridian  altitudes  of  the  sun  with  sextant  and  spirit-level 
horizon.  Observations  of  November  llth  commenced  5  min.  48  sec.  before  apparent 
noon,  and  ended  11  min.  18  sec.  after  noon.  November  12th  commenced  7  min.  23 
sec.  before,  and  ended  5  min.  55  sec.  after  apparent  noon. 


NOVEMBER  11,  1865. 

NOVEMBER  12,  1865. 

•v 

J-L 

"0" 

.a. 

39°  59'  18" 
55'  55" 
57'  05" 
56'  52" 
67'  55" 
55'  52" 
58'  41" 

39°  57'  14" 
57'  36" 
57'  18" 
58'  41" 
59'  43" 
61'  27" 

39°  60'  12" 
59'  28" 
57'  61" 
57'  39" 
66'  01" 
56'  59" 
58'  54" 

39°  54'  19" 
66'  37" 
66'  31" 
58'  13" 
57'  51" 
58'  29" 
59'  01" 

Means,       39°  57'  23" 

39°  58'  38" 

39°  58'  09" 

39°  57'  17" 

These  reductions  have  been  unnecessarily  made  to  seconds  of  arc 
that  the  actual  working  of  the  instrument  may  be  seen ;  the  probable 
error  of  one  observation  deduced  from  these  series  is  one  minute  of 
arc,  and  the  probable  error  of  the  mean  of  all  the  observations  is  thir- 
ty-five seconds  of  arc.  This,  of  course,  excludes  whatever  constant 
errors  may  have  existed. 


The  following  observations  were  made  for  index  error : 

November  11,  1865.  November  12,  18G5. 

Off  the  Arc.  Off  the  Arc. 

_|_  (X  40"        -f-  I'  50"  + 1'  50"        -f  1'  20" 

3    20  2    20  4    20  1    20 

3  20  3    40  2    20  2    50 

4  00  3    10  4    30  1    30 

2  30  1    10  1    50  3    30 

3  40  2    10  2    50  1    50 


Mean,        +2'  39"  Mean,        +2'  30" 

And  the  probable  error  of  one  observation  is  three-quarters  of  a  minute 
of  arc. 

Observations  for  error  of  chronometer  have  been  made  with  the  in- 
strument, and  it  has  been  used  to  measure  the  difference  of  elevation 
of  objects  above  and  below  the  plane  of  the  observer  when  it  was  im- 
possible to  get  their  reflection  in  the  ordinary  artificial  horizon. 

A  thorough  trial  will  be  made  of  this  improvement  this  season,  when 
the  results  can  be  compared  directly  with  the  determinations  of  the 
Coast  Survey. 


